1. Field of the Invention
The invention is concerned with techniques for producing rock wool intended, for example, to serve as a basic material for heat and/or sound insulating products. More precisely, the invention is concerned with an improvement in the technique for fibering a drawable material having a high melting point, for example of the basaltic glass, blast furnace slag or other equivalent types of material in which the material to be fibered, while in the molten state, is poured onto the peripheral strip of centrifuging wheels which are caused to rotate, is accelerated by these wheels, becomes detached therefrom and is transformed partly into fibers under the effect of centrifugal force, a stream of gas issuing tangentially to the peripheral strip on the wheels entraining the resultant fibers towards a receiving means and separating them from the non-fibered material.
2. Description of the Related Art
The fiber producing technique outlined briefly above and known for instance from European patent application Nos. 59 152 and 195 725 is exclusively of the free centrifuging type which means both that the molten glass is not divided into a series of elementary streams (internal centrifuging) nor is it subjected to gaseous drawing by a current of air at elevated temperature and velocity. This known fibering technique produces levels of efficiency and fiber quality far inferior to what can be obtained by other techniques which possibly combine centrifugal processing and gaseous drawing. However it is virtually the only one which can be used economically with materials such as basaltic slag which are characterized by far higher melting temperatures than those of conventional sodo-calcic glasses, a quite steep viscosity-temperature curve and an extremely rapid tendency of devitrification which makes it necessary to work in a very narrow range of temperatures.
As centrifugal processing of the molten glass produces an intense cooling of it, it is considered that in this technique of free centrifugal treatment, the fibers are formed exclusively in a field defined by the surface area of the centrifuging wheel and a concentric fringe at a radial distance of about 5 to 10 mm from the periphery of the wheel. As the fibers are drawn out as soon as there is a break in their connection to the strip of molten glass which adheres to the wheel, this break occurs at a distance which varies considerably from one fiber to another, which explains why it is relatively difficult to define this "fringe" precisely.
In this method of fiber production, the fibers are transported out of the immediate vicinity of the fibering machine by a tangential stream of gas emitted at the periphery of the centrifuging wheels in a direction which is substantially at right-angles to the radial direction of fiber emission. According to the art, this stream is constituted by a flow of cold air or smoke cooled to a temperature close to ambient temperature at a mean velocity of for example 100 m/s, this temperature possibly varying to a fairly wide extent from one installation to another. Such a mean speed is in fact far less than the speed of the non-fibered particles which accompany the fibers and which are created by droplets of glass which splash off after having been accelerated by the rotation of the wheels, but without adhering to them, so that there is no drawing action. Acceleration by the centrifuging wheels imparts to these non-fibered particles sufficient speed that the peripheral gas stream has no significant effect on the trajectory of these particles, which leads to a sorting-out from the fibers which are diverted by virtue of their lesser density and lesser velocity.
In other known methods of producing glass fibers or fibers of some equivalent material, the peripheral gas stream is emitted for other purposes: first of all, one may seek to thin the fibers produced by the centrifuge (in this case there is generally an internal centrifugal process). The gas stream is then emitted by a burner at elevated temperature and velocity, the temperature of the gases having to be much greater than the softening point of the glass in order to produce a drawing of the fibers (see U.S. Pat. No. A-2 577 204 or U.S. Pat. No. A-2 949 632 for example). However, such a flame drawing process presupposes that the material to be drawn has a not very steep viscosity-temperature curve which makes it possible to work in a relatively wide range of temperatures, a condition which is not met by materials such as basaltic glasses or blast furnace slags, its previously indicated. Furthermore, a heated gas flame drawing gas flow is by definition more expensive than a stream of cold air. In other cases, a peripheral gas stream is used which is emitted by a continuous jet at a relatively low temperature (heated vapor at approx. 200.degree. C.) which exerts a very powerful and uniform impregnation on the fibers, the effect of which is to improve their mechanical properties (FR-A-1 169 358). It should be noted that this impregnation effect is in no way justified with materials which are already cooled below their softening point by simple centrifuging effect.
Finally, known from FR-A-2 298 518 and FR-A-2 211 408 is a method according to which one seeks to break continuous filaments by means of a peripheral gas stream emitted at right-angles to their direction of formation, the stream being at a sufficiently low temperature that it does not cause any drawing of the filaments, normally a temperature close to ambient temperature and less than 150.degree. C., and preferably less than 65.degree. C.
In all these known approaches, the gas stream emitted at the periphery of the centrifuging means is therefore either very hot (with temperatures which typically exceed 1000.degree. C.) or cold (at a temperature of either around ambient temperature or around 150.degree. C. when vapor was used--as in the case of the oldest installations which do not have a fairly powerful compressor).